Problem is, its not your pony and most likely hand feed treats are involved, so you stopping it only lasts as long as the owners are away and aren't reinforcing the bad behaviors. I personally HATE and never feed horses any thing by hand ever. But if training not to nip becomes an option heres the trick.
Baked potatoe- If you know how/when to create the nipping behavior, arm yourself with a nice hot baked potatoe. As he comes around to nip make sure he gets a nice bite of that. I've only had 1 horse a confirmed Biter that needed it done twice.
Now to be fair to the pony, if he is getting treats or nipping during girthing because of poor saddle fit. It would be very unfair to punish for a manmade problem.